Atraumatic sensor lead assemblies

ABSTRACT

The invention is directed to lead configurations for sensors that allow for less invasive sensor replacement procedures. In one configuration, a sensor lead assembly includes an outer lead body and an inner lead including a sensor such as an electrochemical glucose sensor. The inner lead can be positioned in an inner conduit of the outer lead body. The outer lead body may be substantially permanently implanted in the patient, and the inner lead can be implanted through the inner conduit of the outer lead body. Once the sensor of the inner lead has worn out or otherwise exhausted its useful life, the inner lead can be removed, and a new inner lead can be implanted in place of the old inner lead.

TECHNICAL FIELD

[0001] The invention relates to implantable medical devices (IMD) and,more particularly, implantable leads that include implantable sensorsfor sensing conditions within a patient.

BACKGROUND

[0002] Sensors facilitate the sensing of certain conditions within apatient. An implantable medical device (IMD) may include circuitryelectrically coupled to one or more sensors to record patient conditionsover time. For example, an implantable lead is typically used to couplea sensor to circuitry within an IMD housing.

[0003] As one example, electrochemical sensors are commonly used fordiabetes patients, e.g., to monitor a patient's glucose levels. In thatcase, a lead can be implanted within a patient to deploy anelectrochemical glucose sensor for sensing levels of glucose in thepatient's blood stream. A distal end of the lead, which includes theelectrochemical glucose sensor, may be positioned in the patient's bloodstream, e.g., within a heart chamber. A proximal end of the lead may becoupled to circuitry within the IMD. The IMD may record the sensedconditions, which may be sent to an external programmer via telemetry,so that a physician can monitor the patient's blood glucose levels.Alternatively or additionally, the IMD may interpret the sensedconditions and possibly deliver therapy to the patient, e.g., bydelivering doses of insulin or drug therapy to the patient if sensedglucose levels exceed one or more therapy thresholds.

[0004] Most glucose sensors use electrochemical methods such as anelectroenzymatic method in which blood glucose is oxidized underglucose-oxidase control, producing gluconic acid and hydrogen peroxide.In that case, the electrochemical glucose sensor can be used to senselevels of gluconic acid and/or hydrogen peroxide in order to estimate apatient's glucose levels. Alternatively, the glucose levels may bedetermined or sensed more directly by the electrochemical glucosesensor. In any case, an electrochemical glucose sensor on a medical leadfacilitates a chemical process which can be identified by electricalsignals to measure a patient's glucose levels.

[0005] In addition to electrochemical glucose sensors a number of othertypes of electrochemical sensors have been developed to measure bloodchemistry or the chemistry of other bodily fluids or material. Ingeneral, electrochemical sensors make use of one or more chemicalprocesses and electrical signals can be used to measure conditionssensed by the chemical processes. Other types of sensors also existincluding pressure sensors, optical sensors, and various other types ofsensors.

[0006] Electrochemical sensors and many other types of sensors, however,typically suffer from stability and longevity problems. For example, theelectrochemical process used by electrochemical glucose sensors cancause significant depletion of the glucose sensors over time, eventuallycausing the glucose sensor to be ineffective for monitoring levels ofglucose in the patient's blood stream. Similarly, other sensors oftenhave similar problems in that prolonged use causes depletion, wear ortissue overgrowth, which can undermine the sensing capabilities of thegiven sensor.

[0007] Sensor replacement is a major concern because replacement ofconventional electrochemical sensor leads is typically a very invasivemedical procedure in which a lead is removed from a patient's bloodvessel, and a new lead is inserted to a high blood flow location, suchas inside a heart chamber. In the replacement procedure, the newelectrochemical sensor lead is typically inserted through the same bloodvessel used for the previous electrochemical sensor lead. Removal of thelead can be very traumatic because of fibrosis around the lead.Moreover, repeated implantation of new leads can cause damage andscarring to the patient's blood vessel, making subsequent implantationmore difficult and more dangerous to the patient. Infection is also aconcern. SUMMARY

[0008] In general, the invention is directed to atraumatic leadconfigurations for sensors that allow for less invasive sensorreplacement procedures. In one configuration, a sensor lead assemblyincludes an outer lead body and an inner lead including a sensor. Thesensor may comprise an electrochemical sensor, or another sensor forwhich replacement during the life of the patient is anticipated. Theinner lead can be positioned within an inner conduit defined by theouter lead body. The outer lead body may be permanently implanted in thepatient and removably attached to an implantable medical device. Theinner lead can be implanted through the inner conduit of the outer leadbody. Moreover, once the electrochemical sensor of the inner lead hasbeen depleted or otherwise exhausted its useful life, the inner lead canbe removed, and a new inner lead can be implanted in place of the oldinner lead. Importantly, this replacement procedure is much lessinvasive and less traumatic than conventional lead replacementprocedures. For example, while the outer lead body remains substantiallyin place, the replaceable inner lead avoids substantial contact andattachment to blood vessel tissue during replacement and use.

[0009] In an alternative configuration, the outer lead body may define awindow that exposes one of a plurality of electrochemical sensorspositioned along axial positions of the inner lead. In that case, when afirst sensor has been depleted, become overgrown with tissue, orotherwise exhausted its useful life, the inner lead may be axiallyrepositioned relative to the window of the outer lead body to expose anew electrochemical sensor. In some cases, this repositioning of theinner lead relative to the window of the outer lead body may beperformed by an implantable medical device (IMD), avoiding are-implantation procedure altogether.

[0010] In one embodiment, the invention provides an electrochemical leadassembly comprising an outer lead body for substantially permanentimplantation in a patient. The outer lead body can be formed with aninner conduit. The electrochemical lead assembly may also include aninner lead positioned in the inner conduit of the outer lead body, theinner lead including an electrochemical sensor positioned in proximityto a distal end of the inner lead.

[0011] In another embodiment, the invention provides an implantablemedical device, such as an implantable sensing device for sensingpatient conditions. For example, the implantable medical device mayinclude a housing including circuitry for storing sensed conditions, andan outer lead body mechanically coupled to the housing. The outer leadbody can be formed with a conduit. The implantable medical device mayalso include an inner lead positioned in the conduit of the outer leadbody and electrically coupled to the circuitry in the housing at a firstend of the inner lead. The inner lead may include an electrochemicalsensor positioned in proximity to a distal end of the inner lead, oranother type of sensor for which replacement is commonly anticipated.

[0012] In another embodiment, the invention provides an outer lead bodyfor substantially permanent implantation in a patent. The outer leadbody may include a first end formed for mechanical attachment to animplantable medical device, and a second end formed with a seal to bepierced by an inner lead. A conduit for receiving the inner lead may beformed in the outer lead body between the first and second end.

[0013] In another embodiment, the invention provides an inner lead. Theinner lead may include a lead body defining a first end and a secondend, and a plurality of sensors formed along axial positions of the leadbody in proximity to the first end. The lead may also include anelectrical contact formed on the lead body in proximity to the firstend, wherein each of the sensors is electrically coupled to theelectrical contact.

[0014] In another embodiment, the invention provides a method comprisingremoving a first lead including a first sensor from an implanted outerlead body, and inserting a second lead including a second sensor intothe implanted outer lead body.

[0015] In another embodiment, the invention provides a method comprisingpositioning an inner lead relative to a window of an outer lead body toexpose a first sensor, and repositioning the inner lead relate to thewindow of the outer lead body to expose a second sensor.

[0016] The invention may be capable of providing a number of advantages.In particular, the invention can greatly simplify, or in some cases,eliminate implantation procedures for electrochemical sensorreplacement. As mentioned above, electrochemical sensor replacement, andother types of sensor replacement is a major concern because replacementof conventional sensor leads is typically a very invasive medicalprocedure in which a lead is removed from a patient's blood vessel, anda new lead is inserted. The invention provides various alternatives toconventional lead replacement that are much less invasive and traumaticto a patient. In this manner, the invention can improve patient care andcomfort, avoid infection, and generally improve therapy to the patient.Moreover, to the extent conventional sensor replacement is a barrier topatient care, the invention may remove the barrier, possibly causingphysicians to be more willing to use sensors in patient care.

[0017] Additional details of various embodiments are set forth in theaccompanying drawings and the description below. Other features, objectsand advantages will become apparent from the description and drawings,and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0018]FIG. 1 is a conceptual diagram illustrating an exemplaryimplantable medical device (IMD) in a human body.

[0019]FIG. 2 is a simplified schematic view of an exemplary IMD with anenlarged view of a distal end of an exemplary implantable lead assembly.

[0020]FIG. 3 is a simplified schematic of first and second ends of anexemplary implantable lead assembly including an outer lead body and aninner electrochemical lead.

[0021]FIGS. 4 and 5 are enlarged cross-sectional views of distal ends ofexemplary outer lead bodies.

[0022]FIG. 6 is a perspective view of one embodiment of an IMD.

[0023] FIGS. 7-10 are cross-sectional side views of embodiments of IMDs.

[0024]FIGS. 11 and 12 are flow diagrams illustrating sensor implantationand replacement techniques.

DETAILED DESCRIPTION

[0025] The invention is directed to lead configurations for sensors thatallow for atraumatic sensor replacement procedures. Various embodimentsof the invention are directed to implantable medical devices (IMDs),lead assemblies, outer lead bodies, inner leads, and various sensorreplacement methods, including improved implantation procedures that maybe executed by a physician, as well as sensor replacement proceduresthat may be executed by an IMD.

[0026] In one configuration, a sensor lead assembly includes an outerlead body and an inner lead including an electrochemical sensor. Theinner lead may be positioned within an inner conduit defined by theouter lead body. The lead assembly may be connected to a housing of anIMD. The outer lead body may be substantially permanently implanted inthe patient, and the inner lead can be implanted through the conduit ofthe outer lead body. Then, once the electrochemical sensor of the innerlead has been depleted, become overgrown with tissue, or otherwiseexhausted its useful life, the inner lead can be removed, and a newinner lead can be implanted to replace the old inner lead with thedepleted sensor. The replacement procedure associated with thisembodiment may be executed by a physician, but may be much less invasiveand traumatic than conventional lead replacement procedures. Forexample, the replacement procedure may be subcutaneous.

[0027] In an alternative configuration, the outer lead body may define awindow that exposes one of a plurality of electrochemical sensorspositioned along axial positions of the inner lead. A physician mayreposition the inner lead relative to the window to expose a newelectrochemical sensor, or alternatively, an IMD may be programmed toreposition the inner lead relative to the window to expose a newelectrochemical sensor, e.g., upon determining that the previouslyexposed electrochemical sensor has worn out. Accordingly, thereplacement procedure associated with this embodiment may be executed bya physician or an IMD. In particular, the IMD may include a motor orother actuator capable of causing axial displacement of the inner lead.If executed by the IMD, the replacement procedure may avoid a subsequentimplantation procedure altogether.

[0028] Many details provided below with respect to electrochemicalsensors may find useful application with a wide variety of other typesof sensors including oxygen sensors, pressure sensors, optical sensors,and many other types of sensors. In many cases, the sensors arenon-electrode sensors, i.e., non-stimulative. If removal or replacementof the non-electrode sensor is desirable, then the teaching herein maybe used to replace the sensor lead in response to such tissueovergrowth.

[0029]FIG. 1 is a conceptual diagram illustrating an exemplaryimplantable medical device (IMD) 10 in a human body 5. A similar devicemay also be used with other living beings. IMD 10 comprises a housing 12containing various circuitry that controls IMD operations and recordssensed events. Housing 12 is typically hermetically sealed to protectthe circuitry. Housing 12 may also house an electrochemical cell, e.g.,a lithium battery for powering the circuitry, or other elements. Thecircuitry within housing 12 may be coupled to an antenna to transmit andreceive information via wireless telemetry signals. Accordingly,conditions sensed by IMD 10 may be sent to an external programmer sothat a physician can examine the sensed conditions for diagnosticpurposes.

[0030] IMD 10 also comprises a lead assembly that includes an outer leadbody 16 and an inner lead 18 positioned within an inner conduit of outerlead body 16. Outer lead body 16 may be implanted through a blood vesselof human body 5 and fed into a heart chamber. An anchoring structure 19may anchor outer lead body 16 in proximity to its entry point into thevenous system. As outlined in greater detail below, outer lead body 16can be implanted in human body 5 in a substantially permanent manner. Inother words, outer lead body 16 may remain implanted within body 5 eventhough inner lead 18 may be withdrawn and replaced from time to time.Accordingly, inner lead 18 comprises a removable lead that can beremoved from the conduit of outer lead body 16 without removing outerlead body 16 from human body 5. In particular, inner lead 18 can beremoved from human body 5 and replaced with a relatively simplereplacement procedure that is much less invasive and much less traumaticthan conventional implantation procedures.

[0031] Inner lead 18 comprises a lead body that includes anelectrochemical sensor in proximity to a distal tip of inner lead 18, oranother sensor element that has a relatively short useful life.Electrochemical sensors refer to sensors that make use of one or morechemical processes and electrical signals. In particular, the electricalsignals may change with certain chemical processes and thereby identifyconditions in a patient. Several types of electrochemical sensors havebeen developed including electrochemical glucose sensors,electrochemical oxygen sensors, and the like. The problem with suchelectrochemical sensors, however, is that they generally have arelatively short useful life. In particular, the chemical processes cancause corrosion and depletion of the electrochemical sensor, whicheventually renders the sensor ineffective for its sensing purposes.Tissue overgrowth may also render the sensor ineffective. When thesensor is rendered ineffective, the electrochemical sensor typicallyrequires replacement. The “outer-inner” lead assembly configurationdescribed herein is one example of a lead assembly that can facilitateless invasive and less traumatic sensor replacement than conventionalimplantation. The invention may find useful application withelectrochemical sensor leads, or other types of sensor leads havingrelatively short useful lives.

[0032] IMD 10 may comprise sensing device that senses conditions ofhuman body 5 and stores the sensed conditions. As mentioned, IMD 10 maysupport telemetry communication with an external programmer in order toallow the sensed information to be delivered to a physician foranalysis. The invention, however, is not necessarily limited to use withpassive sensing devices, but may also find application in devices thatactively deliver therapy to the patient. In that case, the circuitry inhousing 12 may interpret sensed conditions, and deliver therapy to humanbody 5 in response to the sensed conditions. For example, the therapymay comprise therapeutic electrical pulses, drug delivery, or the like.

[0033] In the description that follows, many details of the inventionwill be described with reference to an IMD that performs glucose sensingfunctions for diabetes patients. In that case, the electrochemicalsensor on the distal tip of inner lead 18 would comprise aelectrochemical glucose sensor positioned in a high blood flow locationsuch as in a chamber of the heart. Again, however, the invention is notnecessarily limited to glucose sensors, or even electrochemical sensors,and may find useful application in any sensor applications in whichsensor replacement is commonly required. In some cases, the sensors maybe non-electrode sensors for which tissue overgrowth over the sensor isgenerally undesirable.

[0034] Diabetes mellitus is the most common of endocrine disorders, andis characterized by inadequate insulin action. Diabetes mellitus has twoprincipal variants, known as Type I diabetes and Type II diabetes. Thelatter is also referred to as DM/II (diabetes mellitus type II),adult-onset diabetes, maturity-onset diabetes, or NIDDM (non-insulindependent diabetes mellitus).

[0035] Type I diabetes is usually diagnosed in children and youngadults. In Type I diabetes, the body does not produce insulin. Insulinis necessary for the body to be able to use sugar. When sugar builds upin the blood instead of going into cells, it can cause problems such asa short-term lack of energy, and longer term problems with eyes,kidneys, nerves, the heart, or other organs or systems.

[0036] Type II diabetes generally develops in adulthood, and the risk ofdevelopment of Type II diabetes generally increases with age. Factorssuch as obesity also contribute to the risk. A patient suffering fromType II diabetes secretes insulin, but the insulin's target cells areless sensitive to insulin. Symptoms of Type II diabetes are typicallyslow to appear, and a patient having Type II diabetes may not be awareof his condition. A blood test may show whether the patient has impairedglucose tolerance (IGT), which is often a precursor to Type II diabetes,or compensated Type II diabetes. Unless addressed with treatment such asdiet and exercise, these conditions may develop into uncompensated TypeII diabetes, a very serious condition.

[0037] Patients that have diabetes as well as at patients at risk fordiabetes may use an IMD that includes a glucose sensor, e.g. to monitorlevels of blood sugar. Most glucose sensors use electrochemical methodssuch as an electroenzymatic method in which blood glucose is oxidizedunder glucose-oxidase control, producing gluconic acid and hydrogenperoxide. In that case, the electrochemical glucose sensor can be usedto sense levels of gluconic acid and/or hydrogen peroxide in order toestimate a patient's glucose levels. Alternatively, the glucose levelsmay be determined or sensed more directly by the electrochemical glucosesensor. In any case, an electrochemical glucose sensor on a leadfacilitates a chemical process. Electrical signals can be used toidentify these chemical processes on the electrochemical sensor in orderto identify certain conditions indicative of a patient's glucose levels.Optical glucose sensors are also used in some cases, and may likewisebenefit from the teaching herein.

[0038]FIG. 2 is a simplified schematic view of an exemplary glucosesensing IMD 10A with an enlarged view of a distal end of an exemplaryimplantable lead assembly. IMD 10A comprises a housing 12A which housesvarious circuitry that controls glucose sensing operations and recordssensed events. IMD 10A also comprises a lead assembly 11 that includesan outer lead body 16A and an inner lead 18A positioned within a conduitof outer lead body 16A.

[0039] Outer lead body 16A may be implanted through a blood vessel suchas the superior vena cava of a patient to feed the distal end 20 ofinner lead 18A into a high blood flow location. For example, distal end20 of inner lead 18A may be placed in a heart chamber such as the rightatrium. The distal end 20 of inner lead 18A includes a glucose sensorfor sensing glucose in the patients blood, e.g., an electrochemicalglucose sensor. Again, outer lead body 16A is generally implanted in thepatient in a substantially permanent manner. Inner lead 18A, on theother hand, comprises a removable lead that can be removed from theconduit of outer lead body 16A without removing outer lead body 16A fromthe patient. Accordingly, inner lead 18A can be removed from the conduitof outer lead body 16A and replaced with a relatively simple replacementprocedure that is much less invasive and much less traumatic thanconventional implantation procedures.

[0040]FIG. 3 is a simplified schematic view of first and second ends 31,32 of an exemplary implantable lead assembly 30 including an outer leadbody 36 and an inner lead 38 that includes an electrochemical sensor 39such as an electrochemical glucose sensor. As illustrated in FIG. 3,outer lead body 36 defines a conduit 37 sized to receive inner lead 38.Likewise, inner lead 38 is sized for insertion into the conduit 37 ofouter lead body 36. Inner lead 38 has sufficient rigidity to allowinsertion and removal from conduit 37, and may include a lubricationlayer to improve insertion or removal. A stylet or other structuralenhancement may be used to provide added rigidity to inner lead 38during insertion or removal.

[0041] A first end 31A of inner lead 38 can be electrically coupled toan IMD. A second end 32A (the distal end) of inner lead 38 includeselectromagnetic sensor 39. Electrical signals can be used to measurechemical reactions that occur on sensor 39 in order to measure glucoselevels in a patient. Again, however, electrochemical sensor 39 is onlyone example of senor that may be used on an inner lead in accordancewith the invention. Pressure sensors, optical sensors, biologicalsensors, and a wide variety of non-electrode sensors for which tissueovergrowth is undesirable may benefit from the teaching herein.

[0042] The first and second ends 31B and 32B of outer lead body 38 areformed with sealing mechanisms. In particular, first end 31B of outerlead body 36 is formed for sealed attachment to an IMD. For example,protruding features 41 may be formed on outer lead body 36 in proximityto first end 31 to facilitate attachment to an IMD in a sealed fashionto ensure that body fluid cannot enter conduit 37. The second end 32B ofouter lead body 36 may be formed with a seal 42 that is pierced by innerlead 38 upon insertion of inner lead through conduit 37. By way ofexample, seal 42 may comprise a silicone material to ensure that fluidcannot penetrate into conduit 37.

[0043]FIGS. 4 and 5 are enlarged cross-sectional views of distal ends(second ends) of exemplary outer lead bodies that may correspond toouter lead body 36 of FIG. 3. As shown in FIG. 4, outer lead body 48 isformed with a seal 44 on its distal tip. This seal 44 can be pierced byan inner lead upon insertion of the inner lead into conduit 47. Seal maybe pre-slit with a resealable slit 45 to simplify piercing through seal44. Seal 44 may define a spring force about slit 45 upon piercing slit45 with an inner lead. Accordingly, seal 44 can help ensure that minimalfluid, tissue, proteins, and/or cells are able to enter conduit 47 uponinsertion of outer lead body 48 into a patient and may define a goodhermetic seal upon being pierced by an inner lead. Seal 44 may also bevery useful in limiting or avoiding fibrous tissue growth between aninner lead and outer lead body 48.

[0044] Outer lead body 48 may also include a metal portion 49, such as ametal band, in proximity to the distal end in order to facilitatelocation in the human body via an electromagnetic radiation source.Other radio opaque material may also be used to realize portion 49,including for example, a metal powder molded with the plastic lead body48. In any case, X-rays, radio waves, or other electromagnetic radiationmay be used to pinpoint portion 49 within a patient, allowing aphysician to implant outer lead body 48 in a desired location with greataccuracy. By way of example, portion 49 may comprise a platinum materialwelded to a major portion 46 of outer lead body 48. Major portion 46 mayhave sufficient rigidity to ensure that it will not collapse underpressure of anchoring structure 19 (FIG. 1) or a medical suture used toanchor outer lead body 48 in a location within a patient. A relativelyrigid layer of material (not shown) may be used to form part of majorportion 46 to provide such strength, particularly in locations where asuture would likely be used. Major portion 46 may also include a coiledlayer (not shown) to avoid collapse of major portion if major portion 46it is bent during the implantation of outer lead body 48.

[0045]FIG. 5 illustrates an alterative configuration of outer lead body58 formed with a redundant seal 54 on its distal tip. In particular, theredundant seal 54 defines a plurality of sealing structures 55A, 55B,55C that can be pierced by an inner lead upon insertion of the innerlead into conduit 57. Such redundant sealing can further ensure thatfluid does not enter conduit once sealing structures 55A-55C have beenpierced by an inner lead. Each of the plurality of sealing structures55A, 55B, 55C may be pre-slit with respective resealable slits 56A, 56B,56C to simplify piercing through the respective seal.

[0046]FIG. 6 is a perspective view of an IMD 10C. As shown in FIG. 6,IMD 10C comprises a housing 62 and an electrochemical lead assemblyincluding an inner lead 64 and an outer lead body 66. Housing 62 mayinclude a connector module 65 that facilitates electrical coupling ofinner lead 64 to circuitry within housing 62. Outer lead body 66 isformed with a conduit that mates with inner lead 64. Accordingly, uponimplantation of outer lead body 66, inner lead 64 can be removablyinserted into the conduit of outer lead body 66. Thus, if inner lead 64wears out, a new inner lead can be inserted in its place with a verynon-invasive subcutaneous medical procedure that is less traumatic to apatient than conventional replacement procedures.

[0047] Outer lead body 66 includes a sealing structure 67 to facilitatea sealed engagement with connector module 65. Connector module 65 maydefine a channel 71 through which inner lead 64 can be removed from IMD10C, e.g., by pulling on the proximal end 72 of inner lead 64 to slideinner lead 64 from the conduit of outer lead body 66 and through channel71. In other words, inner lead 64 may be removed from the conduit ofouter lead body 66 through channel 71 of connector module 65. A newinner lead can then be inserted through channel 71 and into the conduitof outer lead body. In that case, outer lead body 66 may not need to beremoved from connector module 65.

[0048] As further illustrated in FIG. 6, the proximal end of inner lead64 may also be formed with a sealing ring 74 to seal against connectormodule 65. In addition, inner lead 64 may also include a mechanical stop76 to ensure that proximal end of inner lead 64 cannot be inserted toofar into connector module 65. An end cap 78 may be provided to cover theproximal end 72 of inner lead 64, e.g., by screwing onto connectormodule 65.

[0049]FIG. 7 is a cross-sectional side view of another embodiment of anIMD 80. In the example of FIG. 7, IMD 80 comprises a housing 82 and aelectrochemical lead assembly including an inner lead 84 and an outerlead body 86. Inner lead 84 defines a lead body that includes aplurality of electrochemical sensors 87A-87C formed along axialpositions of the lead body in proximity to a distal end of inner lead.The proximal end of inner lead may include one or more electricalcontacts (not shown).

[0050] For example, each of the electrochemical sensors 87A-87C may beelectrically coupled to the electrical contact area on the proximal end.In some cases, the electrical contact may define a plurality ofelectrical contact areas formed at axial positions along the length ofthe lead body in proximity to the proximal end. Each of theelectrochemical sensors 87A-87C is electrically coupled to acorresponding one or more of the plurality of electrical contact areas.Electrical components housed in housing 82 can be electrically coupledto electrochemical sensors 87A-87C so that sensed conditions can bestored or logged by the circuitry.

[0051] Outer lead body 86 may be formed with a window 85 in proximity toa distal end of outer lead body 86. Seals (not shown) may seal anexposed portion of inner lead 84 at window 85 from the conduit of outerlead body. In other words, seals may be provided in proximity to window85 to seal an exposed sensor from the conduit and to seal non-exposedsensors from patient fluids and the like.

[0052] One of sensors 87 can be positioned relative to window 85 forsensing purposes. Moreover, IMD 80 may be caused to rotate reel 89 or asimilar structure, to expose a new electrochemical sensor, e.g., sensor87B to window 85 This rotation of real 89 or a similar structure may beperformed in response to a physician activated programming routine,mechanical screwing by the physician, or automatically activatedrotation by IMD 80 upon determining that sensor 87A has worn out. If IMD80 causes automated rotation of reel 89, IMD 80 may include a motor forperforming such rotation. In any case, such repositioning of sensors 87of inner lead 84 relative to window 85 of outer lead body 86 can providesensor replacement with an atraumatic medical procedure, or possibly nomedical procedure.

[0053] FIGS. 8-10 illustrate another configuration of IMD 90. As shownin FIGS. 8-10, IMD 90 comprises a housing 92 and a electrochemical leadassembly including an inner lead 94 and an outer lead body 96. Innerlead 94 defines a lead body that includes a plurality of electrochemicalsensors 97A-97C formed along axial positions of the lead body inproximity to a distal end of inner lead 94. The proximal end of innerlead 94 may include one or more electrical contacts areas 98A-98C. Forexample, each of the electrochemical sensors 97A-97C may be electricallycoupled to a corresponding one of the plurality of electrical contactareas 98A-98C. In any case, electrical components housed in housing 82can be electrically coupled to electrochemical sensors 97A-97C so thatsensed conditions can be stored or logged by the electrical components.

[0054] Outer lead body 96 may be formed with a window 95 in proximity toa distal end of outer lead body. One of sensors 97A can be positionedrelative to window 95 for sensing purposes. When sensor 97A wears out, arelatively simple subcutaneous medical procedure may be performed inwhich a physician accesses housing 92, such as via a small incision intothe patient. End cap 100 is removed as illustrated in FIG. 9, and innerlead 94 is repositioned by a physician relative to outer lead body 96.For example, inner lead 94 may be repositioned relative to outer leadbody 96 to expose a new sensor 97B to window 95, and possibly to exposea new electrical contact area 98B to circuitry within housing 92.Contact area 98A may correspond to electrochemical sensor 97A, contactarea 98B may correspond to electrochemical sensor 97B, and so forth. Anelectrical contact within housing 92 may be positioned such that when anew sensor 97B is positioned relative to window 95, a correspondingelectrical contact area 98B is positioned for electrical contact withcircuitry in housing 92.

[0055] A portion 102 of inner lead 94 may be cut by the physician, andend cap 100 can be replaced as illustrated in FIG. 10. In this manner, arelatively simple electrochemical sensor replacement procedure can beused instead of more invasive conventional lead replacement procedures.

[0056] In still other embodiments, sensors may be positioned at a sameaxial location about the diameter of the inner lead. In that case,repositioning the inner lead with respect to the outer lead body maycomprise rotating the inner lead relative to the outer lead body toexpose a new sensor to a window of the outer lead body.

[0057]FIGS. 11 and 12 are flow diagrams illustrating sensor implantationand replacement techniques. FIG. 11 illustrates a sensor implantationand replacement technique in accordance with embodiments of theinvention in which an IMD similar to those illustrated in FIG. 1, FIG. 2or FIG. 6 is used. By way of example, IMD 10 of FIG. 1 will be used indescribing FIG. 11. As illustrated in FIG. 11, a physician implants anouter lead body 16 in a substantially permanent location of the patient(111). In particular, the physician may insert outer lead body 16through the superior vena cava and feed outer lead body 16 into a heartchamber. The outer lead body 16 may be secured to the patient byanchoring structure 19, e.g., to the connective tissues of the patientin proximity to the entry point of the superior vena cava. One or moresutures may also be used to secure outer lead body 16.

[0058] In order to position outer lead body in a proper location, thephysician may use x-ray or radio wave techniques to locate a metalportion 49 (FIG. 4) of outer lead body 16. The physician then implantsinner lead 18 through a conduit of outer lead body 16 (112). Inparticular, the physician can insert inner lead 18 through the conduitof outer lead body, eventually piercing a seal 44 or 54 (FIGS. 4 and 5)on the distal end of outer lead body 16. A lubricant may be added toinner lead 18 to improve the insertion through the conduit of outer leadbody 16. Alternatively, the insertion of the inner lead into the outerlead body (112) may be performed prior to implantation of the outer leadbody (111). In that case, both outer lead body and inner lead would beinserted into the patient at the same time. In any case, physician mayuse Fluorscopy to visualize a sensor location relative to a distal endof the outer lead body 18. Also, marks or radio opaque material on theinner lead may be used to help the physician location the correctimplantation point.

[0059] Once both the inner lead 18 and outer lead body 16 have beensuccessfully implanted, inner lead 18 and outer lead body 16 can beconnected to a housing 12 of IMD 10 (113). The inner lead 18 may beelectrically coupled to circuitry within housing 12, and outer lead body18 may be coupled to housing 12. Outer lead body 18 may be formed withone or more structures on a proximal end to ensure a tight seal athousing 12, yet still allow for removal and reattachment of outer leadbody 18 to housing 12 at a later time.

[0060] IMD 10 can then be used to sense medical conditions within thepatient (114). Specifically, an electrochemical sensor, such as aglucose sensor, on a distal end of inner lead 18 may be used to senseconditions. Electrical signals may be affected by such chemicalprocesses to identify such conditions. The sensed conditions may bestored by circuitry within housing 12, and possibly telemetricallytransmitted to a programmer so that a physician can interpret themeaning of the sensed conditions and diagnose the patient accordingly.

[0061] Eventually, the electrochemical sensor of inner lead 16 may wearout (yes branch of 115). As mentioned, the relatively short useful lifeof electrochemical leads is a major concern. Once the electrochemicalsensor of inner lead 16 has depleted or become overgrown with tissue, arelatively simple and non-invasive procedure may be performed to replaceinner lead 16. Specifically, the physician can remove inner lead 16 fromouter lead body 18 (116), and a new inner lead can be inserted in theconduit of outer lead body 18 (117). Again, the physician may useFluorscopy to visualize a sensor location relative to a distal end ofthe outer lead body 18. Also marks or radio opaque markers on the newinner lead may be used to help the physician location the correctimplantation point.

[0062] Such a removal and replacement procedure may be much lessinvasive than conventional electrochemical sensor lead replacementprocedures. By leaving outer lead body 18 permanently implanted, newinner leads can be easily inserted. Only a relatively small incision maybe required to access housing 12 so that outer lead body 18 can beunattached, and inner lead 16 can be pulled from the conduit of outerlead body. If desired, mounting structure 19 may be removed during thereplacement procedure and then reattached following replacement of innerlead 16.

[0063] Also, during the replacement procedure it may be desirable toflush outer lead body 18, e.g. with a saline solution following removalof inner lead 16 and prior to insertion of a new inner lead. Inaddition, an anti-inflammatory agent, such as a steroid may be providedon the distal end of the new inner lead, e.g., to help limit or avoidfibrous tissue growth onto the new inner lead.

[0064] An anti-biotic agent may also be used to flush the outer lead tohelp avoid patient infections.

[0065]FIG. 12 illustrates a sensor implantation and replacementtechnique in accordance with embodiments of the invention in which anIMD similar to those illustrated in FIG. 7-10. By way of example, IMD 80of FIG. 7 will primarily be used in describing FIG. 12. As illustratedin FIG. 12, a physician implants an outer lead body 86 in asubstantially permanent location of the patient (121). Like theprocedure of FIG. 11, the physician may insert outer lead body 80through the superior vena cava and feed outer lead body 16 into a heartchamber. The outer lead body 86 may be anchored by anchoring structureand or more sutures may also be used. Outer lead body 86 may include arigid layer in a section where a suture or anchoring structure can beapplied to avoid collapse of outer lead body when a suture is applied.

[0066] In order to position outer lead body 86 in a proper location, thephysical may use x-ray or radio wave techniques to locate a metalportion or other radio opaque material (similar to 49 of FIG. 4) ofouter lead body 86. The physician then implants a multi-sensor innerlead 84 through a conduit of outer lead body 86 (122). Alternatively,the insertion of multi-sensor inner lead 84 into outer lead body 86(122) may be performed prior to implantation of outer lead body 86(121). The physician may use fluoroscopy to visualize a sensor locationrelative to a distal end of the outer lead body 86. Also, marks onmulti-senor inner lead 84 may be used to help the physician location thecorrect implantation point. The proper implantation point may correspondto a location in which sensor 87A aligns with window 85.

[0067] Once both the multi-sensor inner lead 84 and outer lead body 85have been successfully implanted, multi-sensor inner lead 84 can beconnected to circuitry and outer lead body 86 can be connected to ahousing 82 of IMD 80 (123). Outer lead body 86 may be formed with one ormore structures on a proximal end to ensure a tight hermetic seal athousing 82.

[0068] IMD 10 can then be used to sense medical conditions within thepatient (124). Specifically, a sensor 87A proximate to window 85, suchas an electrochemical sensor may be used to sense conditions. The sensedconditions may be stored by circuitry within housing 82, and possiblytelemetrically transmitted to a programmer so that a physician caninterpret the meaning of the sensed conditions and diagnose the patientaccordingly.

[0069] Eventually, sensor 87A may wear out (yes branch of 125). Oncesensor 87A wears out, multi-sensor inner lead 86 can be moved relativeto the conduit of outer lead body to expose a new sensor, e.g. sensor87B, to window 85. For example, a proximal end of lead 86 may be coupledto a reel 89. Reel 89 may be caused to move, such as via programmingsignals sent from a physician, physical rotation by the physician, orpossibly automatic movement caused by IMD 80 itself. In any case,rotation of reel 89 can cause a new electrochemical sensor, e.g. sensor87B to be positioned relative to window. In this manner, sensorreplacement can be greatly simplified.

[0070] Alternatively, rather that use a reel 89, a configuration of IMD90 like that illustrated in FIGS. 8-10 may be used. In that case, aphysician can remove end cap 100 and pull on inner lead 94 by asufficient amount to move sensor 97B into alignment with window 95. Thephysician can then cut portion 102 and replace the end cap 100 asillustrated in FIG. 10.

[0071] A number of embodiments and features have been described. Thetechniques and structures may find useful application in any scenariowhere sensors on medical leads have a relatively shore life span and thepatient condition requires long term monitoring beyond the useful lifespan of the sensor. Many details of the invention have been provided inthe context of an electrochemical glucose sensor. However, the inventionmay find usefulness in a wide variety of sensor applications,particularly non-electrode sensing applications in which tissueovergrowth or depletion cause the useful life of the sensor to belimited. For example, the invention may be useful for electrochemicalsensors, optical sensors, pressure sensors, blood borne biochemical orchemical sensors, lactate sensors used in the coronary sinus to detectischemia, sensors for measuring CKMB, sensors for measuring Troponin I,T and C, sensors for measuring Myoglobin, sensors for measuring BNP(P-type natriuretic peptide), Micro Electro Mechanical Systems (MEMS)sensors or other types of medical sensors for which replacement duringthe life of the patient is anticipated. These and other embodiments arewithin the scope of the following claims.

What is claimed is:
 1. A sensor lead assembly comprising: an outer leadbody for implantation in a patient, the outer lead body defining a firstend, a second end and a conduit between the first and second ends, thefirst end being formed for mechanical attachment to a housing of animplantable medical device; and an inner lead positioned in the conduitof the outer lead body, the inner lead including an sensor positioned inproximity to a distal end of the inner lead.
 2. The sensor lead assemblyof claim 1, wherein the sensor comprises an electrochemical sensor. 3.The sensor lead assembly of claim 2, wherein the second end is formedwith a seal pierced by the inner lead.
 4. The sensor lead assembly ofclaim 3, wherein the seal defines a plurality of redundant sealingstructures each pierced by the inner lead.
 5. The sensor lead assemblyof claim 1, wherein the outer lead body defines a substantially rigidlayer to avoid collapse of the outer lead body in response to a suturetied around the outer lead body during an implant procedure.
 6. Thesensor lead assembly of claim 1, wherein the conduit of the outer leadbody is formed to removably receive the inner lead, and wherein theinner lead is formed for removable insertion into the conduit.
 7. Thesensor lead assembly of claim 1, wherein the outer lead body is formedwith a window in proximity to a distal end of the outer lead body,wherein the inner lead is formed with a plurality of sensors formedalong axial positions of the inner lead in proximity to a distal end ofthe inner lead.
 8. The sensor lead assembly of claim 7, wherein theinner lead is axially repositionable relative to the window of the outerlead body to selectively expose different sensors.
 9. The sensor leadassembly of claim 1, wherein the sensor comprises an electrochemicalglucose sensor.
 10. The electrochemical lead assembly of claim 1,wherein the outer lead body includes a metal portion in proximity to adistal end of the outer lead body.
 11. An implantable medical devicecomprising: a housing including circuitry for storing sensed conditions;an outer lead body mechanically coupled to the housing, the outer leadbody defining a conduit; and an inner lead positioned in the conduit ofthe outer lead body and electrically coupled to the circuitry at a firstend of the inner lead, the inner lead including a sensor positioned inproximity to a second end of the inner lead.
 12. The implantable medicaldevice of claim 11, wherein the housing further includes circuitry forinterpreting sensed conditions.
 13. The implantable medical device ofclaim 11, wherein the outer lead body defines a first end and a secondend, wherein the first end of the outer lead body is formed formechanical attachment to the implantable medical device.
 14. Theimplantable medical device of claim 13, wherein the second end of theouter lead body is formed with a seal pierced by the inner lead.
 15. Theimplantable medical device of claim 14, wherein the seal defines aplurality of redundant sealing structures each pierced by the innerlead.
 16. The implantable medical device of claim 11, wherein the outerlead body defines a substantially rigid layer to avoid collapse of theouter lead body in response to a suture tied around the outer lead bodyduring an implant procedure.
 17. The implantable medical device of claim11, wherein the conduit of the outer lead body is formed to removablyreceive the inner lead, and wherein the inner lead is formed forremovable insertion into the conduit.
 18. The implantable medical deviceof claim 11, wherein the outer lead body is formed with a window inproximity to a distal end of the outer lead body, wherein the inner leadis formed with a plurality of sensors formed along axial positions ofthe inner lead in proximity to the second end of the inner lead.
 19. Theimplantable medical device of claim 18, wherein the inner lead can berepositioned relative to the window of the outer lead body to exposedifferent sensors.
 20. The implantable medical device of claim 18,wherein the inner lead includes an electrical contact that electricallycouples to circuitry in the housing, wherein each of the sensors iselectrically coupled to the circuitry via the electrical contact. 21.The implantable medical device of claim 20, wherein the electricalcontact comprises a plurality of electrical contact areas formed alongaxial positions of the inner lead, wherein each of the sensorselectrically couples to the circuitry via a corresponding one or more ofthe electrical contact areas.
 22. The implantable medical device ofclaim 10, wherein the sensor comprises an electrochemical glucosesensor.
 23. The implantable medical device of claim 10, wherein theouter lead body includes a radio opaque portion in proximity to a distalend of the outer lead body to facilitate positioning of the inner leadrelative to the outer lead body during an implant procedure.
 24. Anouter lead body for substantially permanent implantation in a patentcomprising: a first end formed for mechanical attachment to animplantable medical device; a second end formed with a seal to bepierced by an inner lead; and a conduit formed in the outer lead bodybetween the first and second end for receiving the inner lead, whereinthe seal defines a plurality of redundant sealing structures to bepierced by the inner lead.
 25. The outer lead body of claim 24, whereinthe outer lead body defines a substantially rigid layer in at least asection of the outer lead body to avoid collapse of the outer lead bodyin response to a suture tied around the outer lead body during animplant procedure.
 26. The outer lead body of claim 24, wherein theconduit is formed to removably receive an inner lead, the inner leadincluding an electrochemical sensor.
 27. The outer lead body of claim24, wherein the outer lead body is formed with a window in proximity toa distal end.
 28. The outer lead body of claim 24, wherein the outerlead body includes a metal portion in proximity to the second end tofacilitate positioning of an inner lead relative to the outer lead bodyduring an implant procedure.
 29. A lead comprising: a lead body defininga first end and a second end; and a plurality of electrochemical sensorsformed along axial positions of the lead body in proximity to the secondend; and an electrical contact formed on the lead body in proximity tothe first end, wherein each of the electrochemical sensors iselectrically coupled to the electrical contact.
 30. The lead of claim29, wherein the electrical contact defines a plurality of electricalcontact areas formed along axial positions of the lead body in proximityto the second end, wherein each of the electrochemical sensors iselectrically coupled to a corresponding one of the plurality ofelectrical contact areas.
 31. A method comprising: removing a first leadincluding a first sensor from an implanted outer lead body; andinserting a second lead including a second sensor into the implantedouter lead body.
 32. The method of claim 31, further comprising removingthe first lead through a channel of a connector module of an implantablemedical device.
 33. The method of claim 31, further comprising:unattaching the outer lead body from a housing of an implantable medicaldevice prior to removing the first lead; and reattaching the outer leadbody to the housing of the implantable medical device followinginsertion of the second lead.
 34. The method of claim 31, furthercomprising flushing the outer lead body with a solution to clean theouter lead body following removal of the first lead and prior toinsertion of the second lead.
 35. A method comprising: positioning aninner lead relative to a window of an outer lead body to expose a firstsensor; and repositioning the inner lead relative to the window of theouter lead body to expose a second sensor.
 36. The method of claim 35,further comprising detecting failure of the first sensor prior torepositioning the inner lead relate to the window of the outer lead bodyto expose a second sensor.
 37. The method of claim 35, wherein themethod is performed by an implantable medical device.
 38. The method ofclaim 37, further comprising programming the implantable medical deviceto execute the method.
 39. A glucose monitoring lead assemblycomprising: an outer lead body for substantially permanent implantationin a patient, the outer lead body being formed with a conduit; and aninner lead positioned in the conduit of the outer lead body, the innerlead including a glucose sensor positioned in proximity to a distal endof the inner lead.
 40. A electrochemical lead assembly comprising: anouter lead body for implantation in a patient, the outer lead bodydefining a conduit; and an inner lead positioned in the conduit of theouter lead body, the inner lead including an electrochemical sensorpositioned in proximity to a distal end of the inner lead.